Conventionally, the control of water temperature for household showers is accomplished by manually adjusting the flow rates of hot and cold water so that the mixture achieves the desired temperature. The water temperature may be adjusted by individual hot and cold valves or by a single valve mechanism which controls the overall flow rate of water exiting the shower head and, additionally, the proportions of hot and cold water. For example, the flow rate may be controlled by pulling and pushing the valve handle and rotating the handle clockwise and counterclockwise for temperature control.
When a shower has been idle for a considerable period of time, the water in the water pipes leading from the supply main and from the hot water tank becomes lukewarm. Under these conditions, it takes a fairly long time to establish a desired, steady temperature of the shower water as the lukewarm water is purged from the lines and steady state cold water from the supply main is mixed with steady state hot water from the hot water tank. It is often advisable to turn on the shower for a period of time before entering to avoid the "shock" of water which is too cold or, worse, the danger of scalding by water too hot.
Once a steady state, desired shower water temperature has been achieved, either with no other uses of the household water system occurring or with a steady rate usage occurring, the equilibrium of the system and, thus, the shower temperature can be upset by a change in water usage in the system. For example, the flushing of a toilet connected to the same cold water supply will reduce the cold water pressure available to the shower head, causing the shower water temperature to increase quickly. Similarly, a sudden hot water demand elsewhere in the local system, such as to wash dishes or take a shower in another bathroom, will reduce the available hot water pressure at the original shower with a resultant drop in shower water temperature.
There are a number of temperature regulation devices for liquids which have been developed, not only for shower water temperature control but also for other uses, such as for industrial processes, washing machines, and the like. The earlier types of devices included valves which were controlled more or less directly by mechanical temperature reactive devices. Several known devices employed bimetallic thermostat mechanisms positioned to sense output water temperature and connected by mechanical links and levers to valves controlling the flow of water from hot and cold sources. Others have exploited temperature related expansion and contraction of gases confined in bellows or cylinders with pistons to control the mixing valves. More modern liquid temperature regulation devices have employed electrical temperature sensors connected to amplifiers driving servomotors or connected to digital computers to control the mixing of hot and cold liquids.
Bimetal thermostatic strips generally incorporate joined strips of metals having different temperature-expansion characteristics such that at various temperatures, the strip bends more or less proportionately. Such strips are reasonably accurate over certain ranges and in certain environments as temperature indicating devices, for example, to rotate a needle with respect to a temperature calibration scale or to rotate an electrical contact. However, the accuracy of such a strip in directly controlling a water flow valve subject to substantial water pressure and varying flow rates, without intervening amplification of some sort, is questionable. Accuracy would likely be degraded further, over time, because of possible corrosion and scale incrustation in a hard water system.
Water temperature control devices have been proposed which control mixing valves by bellows or pistons in cylinders containing gases or vapors which exploit the temperature dependence of the pressure of the contained gas or vapor. Such devices are complex since it is necessary to calibrate the temperature variation in the gas pressure to the fluid pressure of the water sources. Since the available water pressure is not necessarily constant, water temperature errors are likely during pressure variations. Bellows or piston based water temperature controls are likely to be bulky in order to provide sufficient volume to generate the force needed to control the water pressure in the water system. Finally, such devices are likely to have slow reaction times and low sensitivity because of the necessity of equilibrating the water temperature to the gas containing structure and the gas itself.
Electronic control systems, both analog and digital, have been used to control shower water temperatures. In general, the electronic water temperature controllers are conventional feedback control systems wherein water temperature is sensed by a circuit element having a parameter, such as resistance, which varies with temperature. The sensed temperature is compared to a set temperature, and motors are activated to operate individual hot and cold water valves or a single mixing valve to decrease the temperature discrepancy. Electronic water temperature regulators, if properly designed, have a higher sensitivity, quicker response time, and greater accuracy than the thermostatic strip controllers and confined gas controllers described above.